Gate valves are popular in industrial applications for several reasons. They allow the full diameter of the flow bore to be opened, thereby efficiently maximizing flow for a flow bore of a particular diameter. Gate valves are also capable of securely closing against fluid flow, even in high pressure applications. Therefore, they are frequently chosen for use in high pressure and corrosive or erosive environments, such as those found in frac, downhole, or subsea applications. In these environments, gate valves may be exposed to high pressures, high temperatures or low temperatures, oil, corrosive fluids, and erosive elements such as frac sand and other particulates suspended in fluid. The necessity of disposing valves deep underground or underwater or connecting them to components that are disposed deep underground or underwater for these applications also makes it difficult, expensive, and time-consuming to repair or replace valves. Further, failure of valves in these applications can have catastrophic results.
FIG. 1 shows a typical prior art frac gate valve. Metal seats 2a and 2b are disposed within seat pockets 4a and 4b formed in valve body 6. The metal seats 2a and 2b surround the flow bore 8. Gate 10 is raised and lowered between the metal seats 2a and 2b. Springs 14a and 14b press the metal seats 2a and 2b against the gate 10. Spring energized seals 16a and 16b seal the metal seats 2a and 2b against the seat pockets 4a and 4b formed in the body of valve 6. The metal seats may prevent fluid communication between the upstream and downstream bores and between the flow bore 8 and the body cavity 12 by forming seals with the gate 10 on one side and with the seat pockets 4a and 4b on the other side.
Erosive elements such as frac sand and other particulates suspended in fluid can cause substantial damage to frac gate valve components, especially at high pressures and extreme temperatures. In particular, particulates may become trapped between the contact surfaces of the seat and the seat pocket. Particulates trapped between the seat and the seat pocket cause two major problems for frac gate valve operation.
First, trapped particulates cause gate drag. Gate drag increases the operating torque required to raise and lower the gate and may increase the time required to move the gate between the fully open position and the fully closed position. Gate drag increases the amount of time that the gate is in a partially open position, the position in which the gate is most susceptible to pitting in corrosive or erosive environments. Pitting can prevent the gate, especially the lower portion of the gate, from sealing properly to the seat assembly. Gate drag also increases the stress placed on the gate, the seat assembly, and the mechanism used to raise and lower the gate. In some instances, gate drag can cause the frac gate valve to become seized and inoperable.
Second, trapped particulates cause seat and seat pocket erosion. The particulates trapped between the seat and the seat pocket erode the contact surfaces of the seat pocket and the seat. Erosion of the contact surfaces forms pits between the seat and the seat pocket that fluid from the flow bore may enter. This fluid may corrode the contact surfaces of the seat pocket and the seat, making the gap between the contact surfaces larger and allowing more particulates and fluid to enter, leading to more corrosion and erosion. Excessive sand buildup may defeat the spring energized seal. Damage to the spring mechanism and the contact surfaces allows fluid and particulates to flow between the seat and the seat pocket. Repairing damage to the seat and the seat pocket requires a large amount of rework to ensure good sealing surfaces. The seat pocket must be machined to remove the pitted surface, forming an “oversized” seat pocket. The oversized seat pocket requires an oversized seat to mate with it. Accordingly, in order to deal with changing sizes of pockets, current seats must be made available in different sizes for valves of a single size. This repair process requires halting valve use and removing the valve, making it undesirable. Further, the selective fit of the seat introduces complexity and potential for error into the assembly process for a gate valve.
Given these shortcomings in current frac gate valves, there is a need for a means to protect the metal-to-metal sealing surfaces of the seat and the seat pocket of frac gate valves from frac sand and other particulates.